Geared motors, configured so that the output rotation of the motor is outputted via a reducer having high transmitting precision, are used as drive components which require high positioning precision in industrial robots and machine tools. FIG. 3 shows an example of a geared motor. A geared motor 101 has a motor unit 102 and a reduction mechanism 103. The reduction mechanism 103 comprises a reducer 104 coaxially connected to a motor shaft 102a of the motor unit 102, and an output shaft 105 coaxially connected to the output side of the reducer 104. A wave gear drive, for example, is used as the reducer 104.
In the geared motor 101, the rotational angle of the output shaft 105 of the reduction mechanism 103 must be controlled with high precision in order to efficiently carry out the positioning and other tasks. To this end, a motor encoder 106 is mounted on the motor shaft 102a. A command is sent to a motor driver 109 in a signal processing circuit 108 so that the output shaft 105 reaches the desired rotational angle on the basis of A, B, and Z phase signals obtained from the motor encoder 106. The motor driver 109 rotatably drives the motor unit 102 in accordance with the received command. In the geared motor 101, the rotational angle position thereof is controlled based on the mechanical origin of the output shaft 105. Therefore, during activation and other such times, the output shaft 105 is returned to the mechanical origin (the origin position).
The motor encoder 106 for detecting the rotational position of the motor unit 102 is commonly mounted on the rear portion of the motor unit 102. If the installation space for the motor encoder 106 could be omitted, the shaft length of the motor unit 102 could be proportionately reduced, and a flatter profile could be obtained.
In cases in which a space for installing the motor encoder 106 in the motor unit 102 cannot be reliably provided, a possible option is to place the motor encoder 106 on the load side. In this case, the output shaft of the reduction mechanism must be lengthened in order to ensure an installation space for the motor encoder 106. When the output shaft is lengthened, adverse effects occur in which the moment rigidity and other characteristics of the reduction mechanism are reduced.